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International Desalination Association BAH03-102 1

THE SELECTION DESIGN, FABRICATION AND PERFORMANCE OFZERON®100 IN SWRO APPLICATIONS.

Authors G Byrne, R Francis, G Warburton. R.J Bullock & C Kuzler1

Presenter: G Byrne

Abstract

Super Duplex Stainless Steels (SDSS) are finding increasing use in high pressure feed pipework systemson SWRO plants and in other desalting applications using membrane technology.

This paper details the corrosion resistance of ZERON 100 SDSS in seawater service, comparing andcontrasting this with other grades of steel commonly used in SWRO applications.

Key aspects of product design that allow for the construction of lower cost, more mechanically efficientpipework systems and vessels are detailed. Again these are compared and contrasted with other steelsused in SWRO plants.

Fabrication practices and procedures are discussed and welding, forming, machining and galvaniccompatibility with other steels are considered.

Specific case studies are presented where ZERON 100 SDSS has been successfully used in small scaleand large scale new build SWRO applications.

Also presented are cases where the alloy has been successfully used to replace SWRO pipeworkconstructed in other grades of stainless steel that have suffered corrosion failure in service.

Finally the successful use of the steel in novel membrane systems on offshore platforms is alsopresented.


I. INTRODUCTION

This paper considers the nature of the alloy chemistry of ZERON 100, its microstructure and theresultant properties that make it attractive to the design engineer for Sea Water Reverse Osmosis(SWRO) applications. Aspects of design and fabrication to optimise its commercial attraction andperformance in service are also detailed.

II. CORROSION RESISTANCE

The diversity of stainless steel grades, the variation in costs and contradictory reports of their resistanceto corrosion in seawater make it difficult for design engineers to establish a settled and undisputedmaterial philosophy. All stainless steels rely for corrosion resistance upon the formation of a Chromiumrich, oxide film on their surface, known as a passive film. Other elements, in particular molybdenum,nitrogen and tungsten are known to make the passive film, more difficult to breakdown in the presenceof the chloride ion. Figure 1 shows that the passive film in most stainless steels is stable over a widerange of potentials. It also shows what elements extend this range of passivity or enhance corrosionresistance [1]. It can be seen that passivity is lost under highly oxidising (transpassive) conditions andwhen active, general corrosion occurs. Increased temperature also has a negative effect on passivity.

F IG U R E 1 S c h e m a tic d ia g ra m s h o w in g th ee ffe c ts o f a l lo y in g e le m e n ts o n th e

c o r ro s io n o f s ta in le s s s te e ls

C u rre n t D e n s ity / C o rro s io n

Pote

ntia

l

Pass

ive

Ran

ge

A c tiv e

T ra n s p a s s iv e C r , M o , N , W

C rC r , N i, M o

C r , N i, W


A convenient, yet empirical, point of entry to differentiate between the corrosion resistance of stainlesssteels in chloride environments has been provided by Truman [2]. Using linear regression analysis ofchemical composition and pitting and crevice corrosion test data, Truman identified a strong correlationbetween a parameter known as the Pitting Resistance Equivalent (PREN) and, both the critical pittingand critical crevicing temperature (CPT/CCT) for a wide range of stainless steels in chloride solution(Figure 2).

The validity of this relationship has been confirmed by many subsequent authors. Kovach and Redmond[3] published an excellent review of the history of this work, rationalizing differences and discrepanciesin test technique, alloy chemistry and seawater conditions and drawing compelling conclusions as to therelationship between PREN and CCT in seawater. By comparing the results of long term crevicecorrosion tests in warm seawater with laboratory results from ASTM G48 Method D corrosion tests,these authors found that only those alloys with a CCT of 35°C or higher in the Method D test resistedcrevice corrosion in warm seawater (Figure 3).

FIGURE 2 Critical pitting temperatureversus PREN for some stainless steels

0

10

20

30

40

50

60

70

80

90

20 25 30 35 40 45 50

PREN (%Cr + 3.3%Mo + 16%N)

Cri

tical

Pitt

ing

Tem

pera

ture

(°C

)

+W

+W

ZERON 100

6Mo Aust.

DP3

Alloy 28904L

Ferralium

ZERON 25

22Cr Duplex

316

FIG UR E 3 P ercen tag e of sites a ttacked vscritica l crevice tem perature in AS TM G 48D

(K ovach e t a l)

0

2 0

4 0

6 0

8 0

10 0

0 10 2 0 30 40 50 60

C C T A S TM G 48D (°C )

Site

s A

ttack

ed (%

)

Ferrit ic Au sten itic D up lex


Further, they were able to relate this behavior to the PREN number of a range of stainless steels anddetermine the PREN level necessary to avoid crevice corrosion (Figure 4).

This means that for seawater service only SDSS with a PREN of 40 minimum and super austeniticstainless steels (SASS) grades with a PREN of 45 minimum should be considered if crevice corrosion isto be avoided. However, this considers only seawater with typical chloride contents of 18-20,000 ppm.On brine rejection circuits this figure may double and we understand developments in membranetechnology may further concentrate the reject brine with chloride. Figure 5 shows that under mildlyoxidising conditions the CPT of Zeron 100 welds remains at about 50°C as the chloride contentincreases from 20,000 mg/l to 100,000 mg/l. The crevice corrosion resistance was displaced to evenhigher temperatures as the chloride content was reduced correspondingly.

Figure 4 Relationship Between PREN andCritical Crevice Temperature (ASTM G48D)

-10

0

10

20

30

40

50

60

0 10 20 30 40 50 60

PREN (%Cr + 3.3%Mo + 16%N)

CC

T A

STM

G48

D (°

C)

Ferritic

Duplex

Austenitic

FIGURE 5 CCT/ CPT versus chlorideconcentration at a potential of +400mV SCE

1,000

10,000

100,000

1,000,000

0 20 40 60 80 100 120

CCT/ CPT (°C)

Chl

orid

e (m

g/l)

CCT (Parent) CPT (Weld)


The performance of high alloy stainless steels and nickel alloys in seawater service is dependent upontheir proper chemical formulation, processing, heat-treatment and finishing.

The proper combination of alloy chemistry and thermo-mechanical processing is of prime importance.However, Shone [4] also found that:-

2.1 Poor pickling was leading to crevice corrosion of alloy 625 that had previously been believed tobe immune to crevice corrosion in seawater. Grubb [5] confirmed the significance of picklingwith respect to 6% Molybdenum alloys similarly .

2.2 Variation in performance between plate and tube products in 6% Molybdenum alloys wasattributable to differences in processing.

2.3 Small differences in alloy chemistry of duplex and austenitic grades was giving rise todifferences in crevice corrosion resistance.

In view of this, designers and specifiers should seek a consistency in supply source of the raw material(bar, billet, plate and seamless pipes). They should ensure all converters of raw material to finished andsemi finished products are approved and qualified to manufacture by the supplier of the steel. Thisshould ensure that all thermo-mechanical processing of the material is compatible with the basechemistry of the raw material provided. Without this level of consistency in material, variation inperformance can arise [6]. Most reputable steel companies dealing with branded alloys appreciate thisand work hard to retain brand integrity across all product forms.

So, purchase specifications should require tests confirming a suitable level of performance. Theseusually involve ASTM G48 Method A corrosion tests, Charpy impact tests and microstructure tests.This approach has been standardized in the form of ASTM A923. However, the scope of this standard islimited to the detection of intermetallics and does not address fitness for purpose.

III. DESIGN

Table 1 shows the composition and mechanical strength of a range of stainless steels used in SWROplants.

NOMINAL COMPOSITION (WT%)ALLOY Fe Cr Ni Mo Cu W N PREN

0.2%PROOFSTRESS(MPa)

UTS(MPa)

316L bal 17 10 2 - - - 24 170 485

904L bal 20 25 4.5 1 - - 35 215 490

6Mo Aust. bal 20 18 6 0.7 - 0.2 43 300 650

Zeron 100(Wrought)

bal 25 7 3.5 0.7 0.7 0.25 >40 550 750

Zeron 100(Cast)

bal 25 8 3.5 0.7 0.7 0.25 >40 450 700


Figure 6 plots the relative localised corrosion resistance as quantified by the PREN number against thestrength level of these steels. From this it can be seen that the family of duplex steels mirrors the familyof austenitic stainless steels in terms of corrosion resistance but with the benefit of 2 x strength and theadded benefit of higher resistance to chloride stress corrosion cracking.

Table 2 shows the allowable stresses for pipework and vessels of SASS and SDSS alloys according toBritish and American vessel and pipe codes.

Table 2.

DESIGN STRESS FOR TEMPERATURES UPTO 40°C (100°F)

MATERIAL PD5500 (formerly BS5500) ASME VIII div 1. ASME B31.3

6% Mo (UNS S31254)

207 MPa (*Note 1) 185 MPa 207 MPa (*Note 1)

ZERON 100(UNS S32760)

319 MPa (*Note 2) 214 MPa (*Note 2) 250 MPa

* Note 1 - Material not listed in code; design stress calculated in accordance with code rules.

* Note 2 - Enquiry case 5500/111 of PD 5500 and code case 2245-1 of ASME VIII.

Taking typical SWRO conditions of 70 bar max pressure and 40°C max design temperature it is possibleto calculate for a given pipe diameter the wall thickness required to contain the pressure. The result ofthis exercise for a range of pipe sizes is shown in Table 3.

FIGURE 6 PREN versus 0.2% proof stressfor some stainless steels

0

100

200

300

400

500

600

20 25 30 35 40 45 50

PREN (%Cr + 3.3 %Mo + 16 %N) / PRENW (%Cr + 3.3 [%M o + %W ] + 16 %N)

0.2%

Pro

of S

tres

s (M

Pa)

Austenitic Duplex

230422 Cr Duplex

Superduplex

316L904L

Super Austenitic


Table 3.PIPE WALL THICKNESS CALCULATED PER ASME B31.3INTERNAL PRESSURE 70 Bars (1015 psi), at 100°F (40°C)

S31254 ZERON 100NPS Nom.

Wall(mm)

Sch (mm) Nom.Weight

Nom.Wall(mm)

Sch (mm) Nom.Weight

Savinglb/ft

1.5 0.921 10S 2.77 2.09 0.763 10S 2.77 2.09 04 2.181 10S 3.05 5.61 1.807 10S 3.05 5.61 06 3.211 10S 3.40 9.29 2.660 10S 3.40 9.29 08 4.180 20 6.35 22.36 3.463 10S 3.76 13.40 8.9614 6.785 20 7.92 45.61 5.620 10 6.35 36.71 8.9016 7.754 20 7.92 52.27 6.423 20 7.92 52.27 018 8.724 30 11.13 82.15 7.226 20 7.92 58.94 23.2120 9.693 30 12.70 104.13 8.029 20 9.53 78.60 25.5322 10.662 30 12.70 114.81 8.832 20 9.53 86.61 28.2024 11.631 30 14.27 140.68 9.635 30 14.27 140.68 0

Weight (in bold) and therefore, cost savings become apparent at the 8" and larger sizes. Some argue thatfor smaller diameter pipes the requirement to use mechanical joints like Victaulic couplings mitigateagainst the use of schedule 10S pipework systems. However, the use of rolled grooves [7], or the use ofschedule 40S pipe “pup pieces”, complete with machined Victaulic grooves and taper bored on the ID tosuit schedule 10S pipe makes the schedule 10S system a reality.

Today, consideration is being given to SWRO systems with pressures up to 100 bar. Large diameterenergy recovery systems with pipework up to 24" NB are being considered. This particular case isdetailed in Table 4 which compares 6% Molybdenum against ZERON 100. As the system pressure andpipe diameter increases the case for ZERON 100 SDSS application becomes even stronger.

Table 4.PIPE WALL THICKNESS CALCULATED PER ASME B31.3INTERNAL PRESSURE 100 Bars (1450 psi), at 100°F (40°C)

S31254 ZERON 100NPS Nom.

Wall(mm)

Sch (mm) Nom.Weight

Nom.Wall(mm)

Sch (mm) Nom.Weight

Savinglb/ft

1.5 1.308 10S 2.77 2.09 1.085 10S 2.77 2.09 04 3.098 40S 6.02 10.79 2.569 10S 3.05 5.61 5.186 4.561 40S 7.11 18.97 3.782 40S 7.11 18.97 08 5.938 20 6.35 22.36 4.923 20 6.35 22.36 014 9.638 40 11.13 63.44 7.991 30 9.53 54.57 8.8716 11.014 40 12.70 82.77 9.133 30 9.53 62.58 20.1918 12.391 40 14.27 104.67 10.274 30 11.13 82.15 22.5220 13.768 40 15.09 123.11 11.416 30 12.70 104.13 18.9822 15.145 60 22.23 197.41 12.558 30 12.70 114.81 82.6024 16.522 40 17.48 171.29 13.699 30 14.27 140.68 30.61


We have come across contractors who design SDSS pipework systems using 6% Molybdenum designstresses, we have come across contractors who have used allowable stresses for vessels (ASME VIII div1) instead of ASME B31.3 to design pipework systems and there are those who build in their ownadditional factor of safety into the coded design stresses for both SDSS and SASS systems. Suchpractices fail to take full advantage from these steels and constitute poor value engineering . We havenot encountered these practices in any other industry that we deal in.

IV. FABRICATION

4. 1 Welding

Fabrication by welding of all high alloy stainless steels should be carried out by qualified weldersworking to qualified procedures. Qualification of procedures should follow the requirements of ASMEIX but be augmented with a corrosion test to ASTM G48A, Charpy impact testing and a microstructurecheck. Similarly welder qualifications should be in accordance with ASME IX and should also beaugmented with a G48A corrosion test and a microstructure check.

The corrosion resistance of welds, in both ZERON 100 and SASS’s can be reduced as a consequence ofa broadly similar metallurgical response to the weld thermal cycle, and similar metallurgical phenomenaoccurring during solidification of the weld metal [8]. Both alloys are susceptible to the formation ofsigma phase if the weldment is too hot for too long. Sigma phase is a brittle, chromium-rich precipitatethat causes the adjacent matrix to be depleted in Chromium, which lowers its corrosion resistance. Earlyassessments of the extent to which corrosion resistance can be diminished by sigma phase formationwere based upon isothermal heat treating of stainless steels in furnaces to generate sigma and thenmeasuring the corrosion resistance. However, it has been shown [9,10] that simulations of this naturesignificantly over estimate the detrimental effect on corrosion resistance of sigma phase formation inreal welds.

Leonard et al [11] systematically examined the effect of intermetallic phases on the corrosion resistanceof SDSS and SASS GTA weldments. The welding conditions examined covered welding arc energyrepresenting typical industrial practice, the high heat input end of the typical range and heat inputs inexcess of manufacturers recommended values (abusive practice) respectively.

Table 6 shows the peak and average volume fractions of intermetallic phases found in each of the welds.Both grades show a tendency to increasing volume fractions of intermetallic with increasing heat inputs.The austenitic grades precipitated more sigma phase than the duplex steels. This is attributable to thehigher alloy 625 type welding consumables used. Overall the level of sigma phase precipitated as aconsequence of these weld thermal cycles was low.

Table 6.Volume Fraction of Sigma Phase (%)ARC

ENERGY(kJ/mm) TYPICAL

(0.9 to 1.6)HIGH

(1.1 to 2.0)ABUSIVE(1.9 to 3.2)

ALLOY PEAK AVE. PEAK AVE. PEAK AVE.

PRECIPITATE SIZE

31254 1.8 1.4 2.1 1.9 2.4 1.8 4µm diaZERON 100 0.2 0.3 0.8 0.6 1.7 0.5 2µm wide x 10µm long

32750 0.8 0.2 2.6 0.3 1.3 0.4 2µm wide x 10µm long


These welds were then exposed to aerated, chlorinated (0.5 to 1.0 ppm) seawater at 40°C for 62 daysand then (following examination) at 53°C for a further 29 days. It should be recognized at this stagethat aerated chlorinated seawater is a highly oxidising, aggressive medium, more so than the chemicallytreated seawater encountered in HP feed pipework in SWRO systems.

The results of the seawater exposure were that none of the weldments exhibited corrosion attack.Moreover, there was no significant difference between any of the welds. Weight loss measurementsindicated corrosion rates of less than 0.001 mm/yr. maximum.

The difference between the corrosion resistance of real welds that contain sigma phase and simulatedsamples containing isothermally transformed sigma is the size and distribution of the precipitates.Isothermally formed sigma particles have time to nucleate and grow under roughly equilibriumconditions. Diffusion processes cause the adjacent matrix to be depleted in Chromium andMolybdenum, locally lowering the PREN of the materials immediately adjacent to the precipitate. Thiszone of depletion increases as the particle size increases. In real welds the thermal cycle is dynamic.Nucleation and growth of precipitates is an incremental process occurring far from equilibrium, across atemperature gradient and is essentially integrated over the relevant temperature ranges of the weldthermal cycle. This has the effect of restricting the growth and diffusion processes in real welds, sominimizing the width of the denuded zone of lower PREN material surrounding the precipitate. Forpitting corrosion to propagate, the depleted zone must be sufficiently large to contain a stable pit. In realwelds this is more difficult to achieve than in isothermal heat treatments. In essence, it is the size of thesigma precipitate that determines its adverse effect on corrosion resistance not its volume fraction.Francis [9] showed that the size of particles formed in isothermal treatment are 7 to 10 times larger thanthe precipitates formed in welds, for the same overall volume fraction.

Taking a semi quantitative approach encompassing diffusion and percolation theory, Francis andWarburton [12] were able to show that for precipitates of about 2µm in diameter (typical for real welds)a volume fraction of around 4% would be necessary before corrosion resistance would be affected. Thistheory was supported by seawater testing of real welds, deliberately manufactured to give a wide rangeof sigma phase volume fraction, as shown in Figure 7.

FIGURE 7 Depth of attack versus sigmacontent in chlorinated sea water at 35°C

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8 10 12 14

Sigma Content (%)

Max

imum

Dep

th o

f Atta

ck (m

m)

Wall Thickness


These findings have subsequently been supported by others [8,11] working in the laboratory and by theperformance of real welds in the field [13,14].

These results and experiences demonstrate that within the range of practical fabrication there is nodiscernible difference between the pitting corrosion resistance of SASS, other SDSS and ZERON 100weldments in seawater. Indeed, should the designer feel it necessary to further optimise the corrosionresistance of the welded joint the option remains to pickle the fabricated spools. Francis et al [15] haveshown that for ZERON 100, this can increase the CPT of welds in seawater by ~17°C for acidimmersion of spools at 55°C and for welds pickled with special acid paste at room temperature. Ofcourse this would require spools to be of a manageable size and have flanged connections for siteassembly to avoid closure welds on site that could not be pickled.

These results and practical experience shows that the level of tolerance of welded joints in SASS andSDSS to welding conditions beyond the manufacturers recommended levels is good. This is inagreement with Shone [4] who also considered practical fabrication implications on corrosion. Thisshould give the design engineer confidence in the robust performance and pitting corrosion resistance ofweldments in both these steels in SWRO applications.

4.2 Forming

Other fabrication issues relate to the forming of branch connections on manifold pipework feeding themembranes. Such branches can be produced as “set-on” arrangements or as cold formed branches witha stand up height of the branch suitable to allow orbital welding. Previously, some contractorsquestioned the credibility of cold forming branches in such high strength steels on a consistent basis.However, the limit is provided by the mechanical capacity of the machine to form the branch and notthe ductility of the steel. Both G48 Method A and electrochemical testing in seawater has shown thatthe cold work induced during forming of these branches does not impair the pitting resistance of thesteel [16]. Infact, Charles [17] has shown that the cold forming behavior of SDSS, as defined in cuppingtests, is very similar to the SASS.

4. 3 Machining

Machinability and guidelines for duplex stainless steel are detailed in a recent IMOA publication [18].Gunn [19] has observed that SDSS and SASS are more difficult to machine than standard grades. In hiscomparison of these alloys though he found no clear trend. For SDSS processes such as end millingwere easier to perform whilst intermittent cutting operations were more difficult to perform whencompared with SASS. In any case both alloys are machinable by all the common processes (sawing,turning, facing, milling, drilling etc) as would be expected for engineering steels.

4. 4 Assembly and Galvanic Compatibility

Assembly of pumps, vessels, pipe spools, valves and instruments does not generally cause a problem.However, care does need to be taken in the selection of gaskets suitable for seawater service that do notpromote crevice corrosion of flange faces. Kain [20] has considered different types of gasket and theirrelative influence of crevice corrosion. He found that gaskets best suited to seawater service werenatural and synthetic elastomer-type gaskets. (Neoprene, Fluorelastomer, Butyl and Nitrile). Thosegaskets more likely to promote crevice corrosion were PTFE and graphite or carbon filled fibre gaskets.


Rogne [21] has noted that those gaskets less likely to promote crevice corrosion were all water absorbentto some degree. It is thought that this absorbed water may dilute the environment inside the crevicemaking it less severe and so less likely for propagation of crevice corrosion. The gaskets that doenhance crevice corrosion can be considered to be of two types. There are those that do not absorbwater (like PTFE), and there are the graphite filled ones, where it is believed that the graphite enhancesthe cathodic reaction to facilitate crevice corrosion. All stainless steels are susceptible to this negativeinfluence on crevice corrosion resistance

Designers should also be sure that when spiral wound gaskets are selected in seawater applications theactual winding material is galvanically compatible with the parent material. For example alloy 400 or316L windings would suffer corrosion. The corrosion products formed are often aggressive with respectto the flange material and can cause corrosion [22]. Compatible windings for ZERON 100 flanges inseawater service are Titanium, SASS, alloy 625 or alloy C276.

Often it may be necessary to construct pipework systems containing different grades of steel i.e. SDSSpumps connected to SASS pipes or dissimilar metal valves or instrumentation in a pipework system. Insuch cases the risk of galvanic corrosion must be addressed. At least three major projects to determinethe galvanic compatibility of dissimilar metal joints between notionally marine corrosion resistant alloyshave been carried out. Shone et al [4] concluded that provided two materials were intrinsically resistantto crevice corrosion in seawater in their own right, then they could be coupled together without risk ofgalvanic corrosion. This view has been subsequently confirmed by Kain [23] and Turnbull [24] and byalmost 20 years of experience of the use of SDSS pumps and valves in SASS seawater cooling systemsused in the Norwegian sector of the North Sea [27].

Table 7 [28] categorizes what material grade couples are galvanically compatible in seawater and thosethat should be avoided or engineered with care.

TABLE 7. Alloy Groupings for Seawater at Ambient Temperature.

CATEGORY TYPE ALLOY1 Noble; passive Nickel-chrome-molybdenum alloys (Mo>7%),

6% Molybdenum austenitic stainless steel, Super DuplexStainless Steel, Titanium and its alloys

2 Passive; not trulycorrosion resistant

Alloy 400/K-500, 904L, 22% Cr duplex, Alloy 825Alloy 20, 316L

3 Moderate corrosionResistance

Copper alloysAustenitic cast ironCarbon Steel / Cast iron4 Poor corrosion

Resistance Aluminium alloys

V. APPLICATION CASE STUDIES

In the early 1990’s the Spanish Ministry of Water were actively seeking materials options for SWROsystems to compete with 904L and 6% Molybdenum grades. Availability of products and costs were themain issues, that they needed to address. The drive was to build plants in the Canary Islands that wouldprovide water for drinking and agriculture and would accommodate the seasonal swell in the populationof the islands during the summer months.


The plants were generally ofmodular construction producing7000 m3/day and capable ofexpansion.(These plants now operateat 27,000 m3/day). Adeje-Arona(Figure 8) was the first plant quicklyfollowed by Playa De Las Americas.Then came Rogue Prieto-Guia, againin Tenerife and then Boca Barrancoin the Gran Canaria. These plantshave all worked well since theirinstallation and deployment ofZERON 100 in similar applicationsaround the world continues.

Figure 8

There have also been several cases where ZERON 100 has been used to replace 316L and 22%Chromium duplex stainless steels that were suffering corrosion in SWRO applications. In the UAE two2,500m3/day facilities and more recently in Saudi Arabia a 14,000m3/day facility, originally constructedin 316L material, have been retrofitted with ZERON 100 pipework The 316L pipe spools had sufferedpitting at welds and in the mother pipe and crevice corrosion at flange faces. The UAE systems werefabricated in the territory while the system for Saudi Arabia was fabricated in the U.K. and assembledon site.

In the Philippines seawater feed pipework had been constructed in 22% Cr duplex steel in two plants,both of which had suffered pitting at welds and in pipes. These systems were retrofitted with ZERON100 in 1995 with no problems reported since.

As well as main seawater feedpipework systems, ZERON 100 hasbeen used for pumps, valves vessels,energy recovery systems andpermiate ports. The largest SWROproject to date to utilise ZERON 100has been the 106,000m3/day TampaBay Water Project, (Figure 9) wherethe large diameter seawater feedpipework has been constructed in thegrade.

Figure 9.

ZERON 100 has also been deployed in a novel application of membrane technology used by theoffshore oil and gas industry. In circumstances where oil wells are becoming depleted and the pressurein the formation is falling, operators revert to secondary recovery processes. One such process involvesinjection of seawater into the formation to repressurize the well and enhance recovery. However, whenthe reservoir contains significant amounts of barium or strontium, seawater injection can cause thecorresponding sulphate salts to be formed as scales. This reduces the permeability of the well and


causes scaling of the tubulars with salts that are difficult to remove [29]. Moreover, McEihiney et al[30] note that injection of seawater into the formation will provide sulphate nutrients that otherwisedormant, sulphate reducing bacteria will metabolise. As a consequence they will form low molecularweight fatty acids (thereby lowering the pH locally) and generate the sour gas H2S. This causes acorrosion problem, can mean that the products are unsaleable and creates a health hazard.

To mitigate against these phenomena, operators can install Sulphate Removal Plant (SRP). These plantspass seawater through nanofiltration membranes that extract sulphate from the seawater, prior toinjection. The process minimizes scaling, helps to optimise oil recovery and avoids the possibility ofreservoir souring.

Figure 10.

These packages are supplied asmodularized units (Figure 10), to suitlift capabilities on offshore platformsor floating production vessels.Essentially, this involvesconstructing and installing filtrationsystems on offshore platforms andvessels, where process piping andmarine code requirements have to berationalized.

ZERON 100 has performed well in this application since the early 1990’s and continues to findapplications in this sector.

V1. CONCLUSIONS

1. The crevice corrosion resistance of ZERON 100 and SASS’s in warm natural seawater is good.A minimum PREN criterion can be applied to maintain an acceptable level of crevice corrosionresistance provided if these steels are properly processed in manufacture.

2. During welding some sigma phase formation can be expected in welds for both SASS’s andZERON 100. However, in the practical welding range this has no effect on corrosion resistance.This constitutes a high tolerance level to variations in fabrication before problems of pitting ofwelds are encountered in SWRO systems.

3. ZERON 100 can be cold formed without loss of pitting corrosion resistance, it is machinable andcan be assembled in connection with SASS’s and other passive alloys without risk of galvaniccorrosion. Assembly with lower category alloys can give rise to corrosion of the less noblematerial.

4. Vessels and pipework can be designed and built in accordance with international codes andstandards. The high mechanical strengths of ZERON 100 allows significant cost savings to berealized especially at larger pipe diameters and at higher system pressures.

5. ZERON 100 has been deployed in numerous SWRO applications around the world with goodsuccess.


V1I. REFERENCES

1. Sedriks A. J. “Metallurgical Control of Localized Corrosion of Stainless Steels”Proc. Conf. Stainless Steels ’87, York, England, Sept 14-16, 1987, P127-137.

2. Truman S. E. “Effects of Composition on the Resistance to Pitting Corrosion of Stainless Steel”.Proc. Conf. UK Corrosion ’87. Brighton, England, Oct 26-28 1987.

3. Kovach C.W. and Redmond J. D. “Correlations Between the Critical Crevice Temperature,PRE Number and Long Term Crevice Corrosion Data for Stainless Steels”.Proc. Conf. NACE 93 New Orleans, USA. March 1993, Paper No. 267.

4. Shone E.B., Malpas R.E. and Gallagher P. Trans Inst. Mar. Eng., 100 part 4 (1988) Paper 14,Page193.

5. Grubb J. F. and Maurer J. R. “Correlation of the Microstructure of a 6% Molybdenum StainlessSteel with Performance in a Highly Aggressive Test Medium”.Proc. Conf. NACE ’95, Orlando, Florida, USA. March 1995, Paper No. 300.

6. Byrne G, Francis R & Warburton G “Variation in Mechanical Properties and CorrosionResistance of Different Alloys within the Generic Designation UNS S32760”. Proc. Conf.Duplex America 2000, Houston, Texas, USA, 29 th Feb – 1st March 2000, Paper 113-125.

7. Francis R, Byrne G & Jones K – “Performance at Reduced Cost; Zeron 100 Super DuplexStainless Steel Sets the Pace”. IDA World Congress on Desalination and Water Reuse, Oct1997. Madrid, Spain, Vol 111.

8. Gooch T. G. “Corrosion Behavior of Welded Stainless Steels”Welding Journal, Vol 75, No. 5, May 1996, pages 135-154.

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VIII. ACKNOWLEDGEMENTSThe authors would like to thank Servicious Y Procesos Ambientales, Covanta and Weir Techna.